1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to so-called "wet" lithographic printing plate constructions that may be imaged on- or off-press using digitally controlled laser output.
2. Description of the Related Art
Traditional techniques of introducing a printed image onto a recording material include letterpress, flexographic and gravure printing and offset lithography. All of these printing methods require a printing member, usually loaded onto or integral with a plate cylinder of a rotary press for efficiency, to transfer ink in the pattern of the image. In letterpress and flexographic printing, the image pattern is represented on the printing member in the form of raised areas that accept ink and transfer it onto the recording medium by impression; flexographic systems, which utilize elastomeric surfaces, have received more widespread acceptance due to the broad variety of compatible substrates and the ability to run with fluid inks. Gravure printing cylinders, in contrast to raised-surface systems, contain series of wells or indentations that accept ink for deposit onto the recording medium; excess ink must be removed from the cylinder by a doctor blade or similar device prior to contact between the cylinder and the recording medium.
In the case of offset lithography, the image is present on a plate or mat as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry printing system, the plate is simply inked and the image transferred onto a recording material; the plate first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to or in conjunction with inking. The ink-repellent fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
If a press is to print in more than one color, a separate printing plate corresponding to each color is required, each such plate usually being made photographically as described below. In addition to preparing the appropriate plates for the different colors, the operator must mount the plates properly on the plate cylinders of the press, and coordinate the positions of the cylinders so that the color components printed by the different cylinders will be in register on the printed copies. Each set of cylinders associated with a particular color on a press is usually referred to as a printing station.
In most conventional presses, the printing stations are arranged in a straight or "in-line" configuration. Each such station typically includes an impression cylinder, a blanket cylinder, a plate cylinder and the necessary ink (and, in wet systems, dampening) assemblies. The recording material is transferred among the print stations sequentially, each station applying a different ink color to the material to produce a composite multi-color image. Another configuration, described in U.S. Pat. No. 4,936,211, relies on a central impression cylinder that carries a sheet of recording material past each print station, eliminating the need for mechanical transfer of the medium to each print station.
With either type of press, the recording medium can be supplied to the print stations in the form of cut sheets or a continuous "web" of material. The number of print stations on a press depends on the type of document to be printed. For mass copying of text or simple monochrome line-art, a single print station may suffice. To achieve full tonal rendition of more complex monochrome images, it is customary to employ a "duotone" approach, in which two stations apply different densities of the same color or shade. Full-color presses apply ink according to a selected color model, the most common being based on cyan, magenta, yellow and black (the "CMYK" model). Accordingly, the CMYK model requires a minimum of four print stations; more may be required if a particular color is to be emphasized. The press may contain another station to apply spot lacquer to various portions of the printed document, and may also feature one or more "perfecting" assemblies that invert the recording medium to obtain two-sided printing.
The plates for an offset press are usually produced photographically. To prepare a wet plate using a typical negative-working subtractive process, the original document is photographed to produce a photographic negative. This negative is placed on an aluminum plate having a water-receptive, anodized (textured) surface coated with a photopolymer. Upon exposure to light or other radiation through the negative, the areas of the coating that received radiation (corresponding to the dark or printed areas of the original) cure to a durable oleophilic state. The plate is then subjected to a developing process that removes the uncured areas of the coating (i.e., those which did not receive radiation, corresponding to the non-image or background areas of the original), exposing the hydrophilic surface of the aluminum plate. Conventional wet plates also typically contain primer layers, which provide better anchorage of the photopolymer to the aluminum substrate.
A similar photographic process is used to create dry plates, which typically include an oleophobic (e.g., silicone) surface layer coated onto a photosensitive layer, which is itself coated onto a substrate of suitable stability (e.g., a primed aluminum sheet). Upon exposure to actinic radiation, the photosensitive layer cures to a state that destroys its bonding to the surface layer. After exposure, a treatment is applied to deactivate the photoresponse of the photosensitive layer in unexposed areas and to further improve anchorage of the surface layer to these areas. Immersion of the exposed plate in developer results in dissolution and removal of the surface layer at those portions of the plate surface that have received radiation, thereby exposing the ink-receptive, cured photosensitive layer.
Although dry printing requires fewer mechanical assemblies and reduced expenditure of consumables, most high-volume offset printing is currently done on wet-running presses. A typical wet printing plate, as noted above, is based on a water-receptive aluminum surface coated with a hardenable oleophilic photopolymer. While such plates have been criticized as causing premature wear on inking and transfer rollers (see., e.g., U.S. Pat. No. 4,054,094 at col. 1, lines 57-63), they nonetheless remain the standard for most of the long-run printing industry due to their durability and ease of manufacture. Indeed, the form and ink rollers ordinarily do not even contact the plate directly, instead making contact with a layer of fountain solution adsorbed on the surface of the plate; that contact layer provides a substantial lubricating effect that counteracts any tendency toward wear.
Rendering a layer of aluminum, which is hydrophilic but fragile in an unstructured or polished state, sufficiently durable to repeatedly accept fountain solution in a printing environment requires special treatment. Any number of chemical or electrical techniques, in some cases assisted by the use of fine abrasives to further roughen the surface, may be employed for this purpose. For example, electrograining involves immersion of two opposed aluminum plates (or one plate and a suitable counterelectrode) in an electrolytic cell and passing alternating current between them. The result of this process is a finely pitted surface topography that readily adsorbs water. See, e.g., U.S. Pat. No. 4,087,341.
A structured or grained surface can also be produced by controlled oxidation, a process commonly called "anodizing." The anodized aluminum plate consists of an unmodified base layer and a porous, "anodic" aluminum oxide coating thereover; this coating readily accepts water. However, without further treatment, the oxide coating would lose wettability due to further chemical reaction. Anodized plates are, therefore, typically exposed to a silicate solution or other suitable (e.g., phosphate) reagent that stabilizes the hydrophilic character of the plate surface. In the case of silicate treatment, the surface may assume the properties of a molecular sieve with a high affinity for molecules of a definite size and shape -- including, most importantly, water molecules. The treated surface also promotes adhesion to an overlying photopolymer layer. Anodizing and silicate treatment processes are described in U.S. Pat. Nos. 3,181,461 and 3,902,976.
Textured chromium surfaces also exhibit substantial hydrophilic character, and can be used in lieu of aluminum in wet-running lithographic plates. Such surfaces can be produced by, for example, electrodeposition, as described in U.S. Pat. No. 4,596,760. As used herein, the term "textured" refers to any modification to the surface topography of a metal plate that results in enhancement of hydrophilic character.
Although chromium and stabilized aluminum grain surfaces exhibit good durability characteristics during printing, their hydrophilic character also renders them hygroscopic. Excessive sorption of moisture facilitates ongoing chemical reaction that may result in reduction or elimination of hydrophilic character. For this reason, if plates having such surfaces are to be stored, they typically first receive a coating of a protective, water-soluble polymer in a process known as "gumming." On the other hand, as discussed below, the ease with which hydrophilicity is lost provides a basis for digitally controlled, point-by-point imaging of metal-based lithographic plates.
The desire for electronic alternatives to traditional photographic platemaking processes stems from the time, expense, equipment requirements and environmental compliance measures associated with the latter. Recently developed computer-controlled imaging systems, some of which can be utilized on-press, alter the ink-receptivity of blank plates in a pattern representative of the image to be printed. Such imaging devices include sources of electromagnetic-radiation pulses, produced by one or more laser or non-laser sources, that create chemical changes on plate blanks (thereby eliminating the need for a photographic negative); ink-jet equipment that directly deposits ink-repellent or ink-accepting spots on plate blanks; and spark-discharge equipment, in which an electrode in contact with or spaced close to a plate blank produces electrical sparks to physically alter the topology of the plate blank, thereby producing "dots" which collectively form a desired image (see., e.g., U.S. Pat. No. 4,911,075).
For example, as described in U.S. Pat. Nos. 4,947,750 and 4,958,563, intensively heating a grained aluminum or chromium surface transforms that surface from a hydrophilic to a hydrophobic, oleophilic state. Therefore, by selectively exposing a printing plate bearing such a surface to heat, it is possible to create on the plate surface a pattern of ink-receptive image points corresponding to a desired image. Because unexposed surface regions remain hydrophilic, the result is a fully imaged lithographic plate that may immediately be used for printing without the need for chemical processing. Suitable point sources of heat for such plates include spark-discharge and laser equipment.
Indeed, because of the ready availability of laser equipment and their amenability to digital control, significant effort has been devoted to the development of laser-based imaging systems. Early examples utilized lasers to etch away material from a plate blank to form an intaglio or letterpress pattern. See, e.g., U.S. Pat. Nos. 3,506,779; 4,347,785. This approach was later extended to production of lithographic plates, e.g., by removal of a hydrophilic surface to reveal an oleophilic underlayer. See, e.g., U.S. Pat. No. 4,054,094. These systems generally require high-power lasers, which are expensive and slow.
Other laser-based systems for imaging hydrophilic plates operate by removal of inorganic chalcogenide (see, e.g., U.S. Pat. No. 4,214,249) or organic polymer (see, e.g., U.S. Pat. Nos. 5,339,737 and 5,353,705 layers, which are hydrophilic, from an oleophilic substrate such as polyester. Again, while use of a removable hydrophilic surface coating was characterized in the '094 patent as superior to the traditional construction based on a hydrophilic substrate, it nonetheless remains outside the mainstream of conventional printing.
Given the ease with which the hydrophilic structure of a grained-metal plate is disrupted, laser-based imaging systems that operate by etching or ablation ordinarily can be utilized with such plates only by selective destruction of hydrophilic character. This is the case in the '750 patent mentioned above, where the metal is transformed directly, and also in the '094 patent and in U.S. Pat. No. 4,063,949, where the laser is used to melt or slag an overlying polymer into the grained surface, filling the topography and thereby transforming it into an oleophilic surface. Laser ablation of an overlying oleophilic polymer layer to reveal a grained, hydrophilic metal layer thereunder -- resulting in a plate equivalent to the conventional photopolymer-based construction -- has not, as far as we are aware, heretofore been possible. Either the polymer would partially melt, clogging the metal surface grain and rendering it hydrophobic as described in the '094 patent, or, if the laser were operated at power levels sufficient to ensure complete polymer ablation, its energy would physically transform the surface and render it hydrophobic in the manner of the '750 patent.